Lighting panels in the form of a plate with multiple microlenses are known for use in luminaires, to hide the light source (such as a fluorescent tube or LED) and to deliver a preferred light distribution. The preferred light distribution can depend on national guidelines. For example, a microlens optics plate is widely used in Europe and acrylic plates are widely used in North America. Typically, these are made by hot embossing which is a relatively costly batch process (highly dependent on the base material) and limited to plate materials. Three dimensional lenses or exit windows beyond a single curve are therefore hard to obtain.
WO 2005/083317 describes a microlens optics plate for use in luminaires. The plate is a transparent substrate with tapered protrusions that end in a sharp point or in a sharp edge. Most of the light enters the plate via the flat side and exits via refraction at a tapered conical surface. The plate transmits part of the light from a diffuse source within a limited intensity cone (the intensity distribution required to comply with EU regulations) and recycles the remaining part back to the source, where it is reflected and has a second chance to pass the plate.
In designing office luminaires there are a number of optical problems that have to be taken into account.
The actual preferred light intensity distribution depends on the region. In Europe, office luminaires have to comply with EN-12464-1 which prescribes certain horizontal task illumination Lux levels and has strict glare rules that limit the luminaire luminance at large angles with respect to the normal.
In North America the glare limits are less strict, which allows for more light at large angles. The advantage of this is a larger luminance spacing (giving lower cost) and more vertical luminance (better lighting of the walls). The drawback is a lower utilisation efficiency (less light to the task area) and more glare.
Especially for LED lighting, it is necessary to reduce the brightness of the LED source. The high brightness of LEDs can give discomfort glare. High peak brightness typically means a dotty but efficient product. High brightness is especially problematic under angles that are directly in the field of view of an office worker. The luminaire directly above a person is not directly visible and can therefore have a higher brightness than further away when it is within a person's visual field. Typically, the visual field of a person looking straight ahead starts at about 35 degrees with respect to the vertical orientation. Therefore, there is no direct glare from luminaires at viewing angles between 0 and 35 degrees with respect to the vertical. In general, the discomfort glare by a high luminance source in the ceiling increases with increasing angle to the vertical plane (assuming that the people in the room look straight ahead or downward to a screen or desk).
Thus, high peak brightness should be avoided at large angles to the vertical (i.e. for rays with a large vector component in the plane of the luminaire exit window and ceiling), but higher values may be permitted at low angles (i.e. for rays directed downwardly from the ceiling). Since high homogeneity comes at the cost of decreasing optical efficiency, generally requiring more light recycling, it is preferable to homogenize the luminance only at higher viewing angles where it is really required.